Hy sys.executable may confuses python library

[TristanCacqueray]

For example this library: https://github.com/glumpy/glumpy/blob/master/glumpy/ext/glfw.py#L139 uses "sys.executable -c" with python code, which is not working with hy and results in a import error.

Perhaps the -c argument could check the command's dialect and fallback to hy.sys_executable if it's not hylang? Alternatively doing (setv sys.executable hy.sys_executable) fixes this issue.